An oscillator circuit is an electronic circuit for providing an oscillating output signal (frequency signal), such as a clock signal. In order to provide a reference frequency signal, crystal oscillators are used, which use a quartz crystal to provide a reliable frequency signal. Such a reference frequency signal is often used in electronic systems as a standard clock signal which is taken as a basis for producing other frequency signals, for example using a phase locked loop.
Particularly in mobile systems, such as in mobile radio systems, the switching-on time of the oscillator circuit is an important parameter. This becomes clear from the GSM mobile radio system. The GSM mobile radio system uses a time slot method (time divisional multiple access or TDMA) for transmission. In the TDMA transmission scheme, the mobile terminal is alternately operated in an active state and in a quiescent state. In the active state, the terminal is in a transmission or reception state, so that transmission takes place via the communication channel. In this case, the mobile terminal performs signal processing. The signal processing requires a reference clock frequency of 26 MHz, as stipulated by the standard. In the quiescent state between two active states, it is merely necessary to accurately monitor the period of time, so that the mobile terminal can revert to an active state at the right time. According to the stipulations of the standard, the mobile clock signal must operate at a clock frequency of 32 kHz.
To this end, the mobile terminal usually has a relatively simple 32 kHz oscillator circuit and a more accurate 26 MHz oscillator circuit. The 26 MHz oscillator circuit is switched on before a corresponding active state starts, that is to say prior to an actual transmission or reception state. Usually, the switching-on process is between 5 and 10 ms. The longer the switching-on process lasts, the more power required by the 26 MHz oscillator circuit. The frequent changing between active states and quiescent states in the 26 MHz oscillator circuit allows the power consumption of the 26 MHz oscillator circuit and hence of the mobile terminal to be reduced as a result of a reduction in the switching-on time. Overall, a reduced power consumption allows the power reserve of the mobile terminal to be reduced and hence the operating time of the mobile terminal to be extended.